1. Field of the Invention
The present invention relates to a glass composition that is used for covering electrodes and a glass paste containing the same.
2. Related Background Art
Recently, flat panel displays, such as plasma display panels (hereinafter also referred to as “PDPs”), FEDS, and liquid crystal displays, have been gaining attention as displays that can achieve reductions in thickness and weight.
These flat panel displays each are provided with a front panel and a back panel, each of which includes a glass substrate and components disposed thereon. The front panel and the back panel are arranged to oppose each other and the peripheries thereof are sealed.
As described above, a PDP has a configuration in which the front panel and the back panel are arranged to oppose each other and the peripheries thereof are sealed with sealing glass. The front panel includes a front glass substrate. It also includes stripe-like display electrodes that are formed on the surface thereof, as well as a dielectric layer and a protective layer that further are formed thereon. The back panel includes: a back glass substrate; stripe-like address electrodes that are formed on the surface thereof; a dielectric layer that further is formed thereon; barrier ribs that are formed between adjacent address electrodes; and phosphor layers, each of which is formed between adjacent barrier ribs.
The front panel and the back panel are arranged so as to oppose each other and to allow the electrodes thereof to be orthogonal to each other. In this state, their peripheries are sealed. The sealed spaces that are formed inside are filled with a discharge gas.
Two display electrodes compose a pair. The region defined by such a pair of display electrodes and one address electrode that cross each other three-dimensionally, with a discharge space being interposed therebetween, serves as a cell that contributes to an image display.
Hereafter, the dielectric layer of the PDP is described in detail. The dielectric layer of the PDP is required to have the following: higher insulation to allow it to be formed on electrodes; a lower dielectric constant to achieve lower power consumption; and a thermal expansion coefficient that matches that of the glass substrate so that neither peeling nor cracks occur. Furthermore, in order to use the light emitted from phosphors as display light efficiently, the dielectric layer to be formed on the front glass substrate usually is required to be amorphous glass having high visible light transmissivity.
The dielectric layer is formed by applying a glass paste onto a glass substrate by, for example, screen printing and then drying and baking it. The glass paste usually contains glass powder, resin, and a solvent and also may contain an inorganic filler and an inorganic pigment in some cases. On the other hand, from the viewpoints of the price, availability, etc., generally soda lime glass produced by a float process is used as the glass substrate to be used for the PDP. Accordingly, the glass paste is baked at a temperature of 600° C. or lower that causes no deformation of the glass substrate.
Since the dielectric layer that is used for a PDP has to be baked at a temperature that causes no deformation of the glass substrate, it is necessary to form it with glass having a relatively low melting point. Hence, PbO—SiO2-based glass whose main raw material is PbO is used mainly at present.
Such a dielectric layer of the PDP is formed by baking a glass paste containing resin and a solvent. Accordingly, the dielectric layer may be colored by carbon-containing impurities that remain therein, which may cause a deterioration in luminance. For the purpose of preventing such a deterioration in luminance, glass for covering transparent electrodes has been proposed that is obtained by adding MoO3 or Sb2O3 to glass containing PbO (for instance, see JP2001-151532A).
Furthermore, with considerations given to environmental problems, a dielectric layer that is free from lead has been developed. For example, a dielectric layer produced using Bi2O3—B2O3—ZnO—R2O-based glass (R: Li, Na, K) has been proposed (for instance, see JP2001-139345A). Moreover, when glass containing an alkali metal oxide is used, in order to reduce the pinholes that are produced by baking the glass on aluminum electrodes, glass that contains CuO, CoO, MoO3, or NiO added thereto has been proposed (for instance, see JP2002-362941A).
As described above, the dielectric layers produced using glass that is free from lead have been proposed conventionally. In such cases, however, the dielectric layer or front glass substrate may yellow due to the alkali metal oxide or bismuth oxide that is used instead of lead to obtain a lower softening point. Conceivably, the mechanism that causes this yellowing is as follows.
Ag or Cu is used for the display electrodes to be provided on the front glass substrate and the address electrodes to be provided on the back glass substrate. The Ag or Cu may ionize and then may dissolve and diffuse in the dielectric layer and glass substrate during the baking that is carried out in forming the dielectric layer. The diffused Ag ions or Cu ions tend to be reduced by alkali metal ions or bismuth oxides that are contained in the dielectric layer or Sn ions (bivalence) contained in the front glass substrate. In that case, they will colloidize. When Ag or Cu has colloidized as described above, the dielectric layer and front glass substrate are colored yellow or brown, i.e. so-called yellowing occurs (for instance, J. E. SHELBY and J. VITKO. Jr Journal of Non-Crystalline Solids vol. 50 (1982) 107–117). Since such yellowed glass absorbs light with a wavelength of 400 nm, a PDP produced using the glass has a deteriorated blue luminance or deteriorated chromaticity. Hence, yellowing is a problem especially in the front panel. Furthermore, since Ag and Cu colloids have conductivity, they lower the withstand voltage of the dielectric layer. In addition, since Ag and Cu colloids deposit as colloidal particles that are far larger than ions, they reflect the light that passes through the dielectric layer and thereby cause the deterioration in luminance of the PDP.